1. Field of the Invention
The present invention relates to a separately excited inverter circuit and a liquid crystal display (LCD) television, and in particular, to a separately excited inverter circuit with a full-bridge switching circuit and an LCD television whose backlight is illuminated by the separately excited inverter circuit.
2. Description of the Related Art
Electric and electronic appliances including an LCD television are legally obligated to take a short- and open-circuit test for safety. The short- and open-circuit test is conducted such that all the terminals of electric and electronic components are short- and open-circuited within AC voltage ±10%. The test is made to ascertain whether what abnormality is generated in a circuit.
It is needless to say that the test applies to a switching element (including a transistor and an FET) forming a switching circuit in the inverter circuit. The short circuit damage of any switching element in the switching circuit influences other switching elements forming the switching circuit to be liable to damage all the switching elements forming the switching circuit.
Hitherto, abnormality in an inverter circuit has been detected by a software control using a microcomputer. For example, a microcomputer monitors the current and voltage of a fluorescent tube connected to the inverter circuit to determine duration if the microcomputer detects an abnormal decrease in current and an abnormal increase in voltage. If the duration exceeds a predetermined time (for example, 350 milliseconds), the microcomputer determines the excess to be abnormal to shut down the inverter circuit. It has been essential to determine the duration to avoid malfunction.
However, it takes a very short time (for example, 20 microseconds to 40 microseconds in an oscillation of 50 kHz) to damage other switching elements after one switching element of the switch circuit has been short circuited and damaged, so that the microcomputer determining an abnormal voltage has been too late to prevent the short circuit damage.
As circuits for protecting a circuit at the time of the occurrence of abnormality in it there have been known circuits described in the following documents.
Japanese Patent Application Laid-Open No. 05-299990 describes that an avalanche diode and a thyristor are used to protect an FET from overvoltage surge of voltage inputted to the drain of the FET. That is to say, the surge voltage inputted into the drain of the FET is detected by the avalanche diode and the thyristor to the gate of which the detected signal is inputted is momentarily turned on to output the driving electric power to the gate of the FET.
Japanese Patent Application Laid-Open No. 2003-179472 describes that a protective circuit unit detects current flowing between the drain and the source of an FET through a current detecting unit to adjust the output voltage of a charge pump unit adjusting the gate voltage of the FET according to the detected result. The circuit suppresses current outputted from the drain of the FET to protect the element supplied with current through the FET.
Japanese Patent Application Laid-Open No. 2002-353795 describes that a circuit for protecting a switching element from overcurrent is configured such that a MOSFET is turned on when current flowing into the current detecting terminal of the switching element increases and the source of the MOSFET is connected to the gate of the switching element.
None of the techniques described in the abovementioned documents assume that a plurality of switching elements is provided unlike the present application and prevent other switching elements from being damaged owing to the short circuit damage of one switching element.